Bricin, in my experience there's no (verbal) communication code among cyclists here.

1) If a car overtakes a cyclist he's actually required by law to keep a one meter distance. This is a good rule for an overtaking cyclist too - you should only overtake if you can do so without disrupting the other cyclist. Of course you should check the road for cars and motorbikes before leaving the bicycle lane, and give a hand sign.

2) I don't think that cyclists who don't know each other cooperate for drafting. (But I do agree that one should not do drafting without notification, it is irritating among friends).

3) It's very polite to offer help. However I usually have neither spare parts and tools when I go biking, nor exceptional repair skills. So I only stop if an accident happened and somebody could be injured, or if it is obvious that the cyclist with the broken bicycle is a complete newbie.

4) In dangerous situations I keep my hands on the handlebar for braking and manoeuvres. Regrettably not all bicycles do have one, but a bike bell is the best solution here. The only (non-offensive ) hand sign that is universally understood is to indicate with an arm in which direction you're going.